I'm trying to diagnose a problem with my 12 yr old Myson Apollo boiler. Standard setup with rads and a tank. The problem is best described as a loud expansion like "ticking" noise about once a week when the thermostat is set to high. If I set it to low the boiler just cycles every 15 to 20 secs. I imagine its a thermostat problem but any pointers would be very appreciated. I should add that the system seems to be working just fine.